Method of and apparatus for drying substances in vacuo



May 2, 1939.

J. G. w. GENTELE 2,156,845

METHOD OF' AND APPARATUS FOR DRYING SUBSTANCES IN VACUO Filed March 22,1938 J- G. W. Gien-ale Patented May 2, 1939 PATENT OFFICE METHOD OF ANDAPPARATUS FOR DRYING SUBSTANCES IN VACUO Johan Georg Wilhelm Gentele,Stockholm,

l Sweden Application March 22, 1938, Serial No. 197,485

In Austria March 27,1937

1 Claim.

My present invention refers to a method and an apparatus for drying invacuo substances liable to change under the action of heat. Suchsubstances are vegetables, berries, fish, eggs,

5 milk, etc.

My present invention refers to an improvement of the method andapparatus set forth in my copending application Ser. No. 41,486.

As set forth in said earlier application it has hitherto, indryingsensitive substances lying on metallic supports, been necessary tomaintain a relatively low temperature in the heating elements, such asat most 'l0-90 C. in order to avoid burning and destroying of theproducts to be dried when the supports are directly or indirectly heatedby conducted heat. Due to the necessity of employing such lowtemperatures` the drying period has been very long, for instance 24-48hours. Such a long drying period at the low temperature existing in theproducts to be dried in vacuo may cause great inconveniences, such asdestroying of the vitamines, fermentation processes etc.

According to the invention covered by my co- 25 pending application itis possible to dry products of the kind described in a very short time,such as 1 2 hours if the same thickness of the layer of products is usedas in the above mentioned example. This is due to the fact that it ispossible to have the heating elements Aheated to very high temperatures,at least 200 C. and up to 600 or 800 C. or still more, Without damagingthe products. The employment of such high temperature on the heatingelements according to my copending application is rendered possible bypreventing heat to be conducted to the metallic supports for theproducts, which is effected by carefully heat insulating said supportsfrom the conductive parts of the apparatus and from the heatingelements.

The heating of the metallic support and the goods lying on the same istherefore effected practically exclusively by radiant heat emanatingfrom heating elements located above and 45 beneath each support and at adistance from the same. The heating elements which may be heated bysteam, combustion gases, electric current, etc., are of substantiallythe same size as the supports so that said supports and the prod- 50ucts lying on the same are uniformly heated all over their surfaces.

Applicants further investigations have shown that improved results maybe obtainedif the products to be dried are uniformly heated both 55 fromthe upper and from the lower side. It has,

namely, been found that if the supports used are made of bright metaland these are .exposed to radiant heat from the upper and lower side therays coming from the upper side and irnpinging on the products lying onthe supports 5 will penetrate, said products so that the liquid in theupper part of the same is rapidly evaporated and expelled. The rayscoming from the elements beneath the supports and impinging on the lowersurface of said supports will to a great 10 extent be reflected backfrom the same, wherefore only a small part of the rays will be obsorbedby the support so as to heat the same. This' causes that the productsare exposed to a higher temperature on the upper side than on the side15 being in contact with the support. l

In order to avoid this inconvenience when drying substances of the kinddescribed in vacuo applicant has proposed to treat the lower side of themetallic support and, if necessary, also the 20 upper one, so as to getmore or less high ability of absorbing the heat rays. They may, forinstance, be made black or dull or otherwise treated, so that thereflection of heat rays will be avoided as much as possible. 25

The metals adapted to be used as supports in applicants invention shouldthus be treated so as to get a high intensity of radiation. Such metalsare for instance oxidised sheet iron, aluminium sheets which areblackened or made 30 dull on one or both sides so as to get ahighconstant ofradiation, oxidised copper etc. Aluminium is consideredto be the most convenient material since it has no chemical influence onthe products to be dried. 35

By using such supports the greatest part of the rays emitted from theheating elements located below the supports Will rapidly heat thesupports, so that the products lying on the same will beheated frombelow practically as much 40 as they are heated from the upper side,whereby the products will consequently be heated substantially uniformlyfrom both sides In spite of the high temperature of the heating elementsthe supports will, due to the above mentioned heat insulation, bemaintained at a relatively low temperature as long as there is somemoisture in the product. Due to the fact that prac tically no heat willbe conducted to the supports but the heating is effected practicallyexclusively by radiant heat the temperature of the products will not behigher than that which corresponds to the boiling point of the liquidcontained in the products at the existing vacuum. The products arethereby safely prevented from being burnt or destroyed. I! no insulationwere used heat would-be conducted to the supports from the heated wallsof the apparatus in which case the supports would after a short time getsubstantially the same temperature as the heating elements. If,therefore, the temperature of the latter is very high, the products tobe dried will be destroyed. Theheating should be interrupted before allthe moisture has been removed, for instance when the content of moisturehas been reduced to about 5-7%. During the last stage of the dryingprocess it is also convenient to stepwise reduce the supply ofheat tothe heating elements.

In executing the method according to applicants present invention it isconvenient to use an apparatus of substantially the same construction asthat shown in my zopending application. 'Ihe annexed drawing shows bywayof example the same apparatus as is shown in Figs. 1 and 2 of thecopending application.

Fig. 1 is a longitudinal section through the apparatus and Fig. 2 is across-section along the line II-II in Fig. 1.

The apparatus consists of a casing I which may be provided on theoutside with a heat-insulation 2. One of the end-walls is in the form ofa. door 3 which can be opened when products are to be put in or takenout and which is kept hermetically closed during the drying process.Inside the evacuated chamber A there are electrical heating elements 4,which are well-insulated, not only against electric current but alsoagainst heat so that the conduction of heat to the other parts of thevapparatus is prevented. Between the heating elements and at a suitable.distance from them the supports 5 for the products to be dried arelocated, said supports bey ing insulated against conducted heat from theother parts of the apparatus.

The supports may as mentioned consist of for instance oxidised sheetiron, blackened or dull, aluminium or other convenient metal with highintensity of radiation. According -to applicants present inventionespecially the lower side of said supports should be blackened ortreated with acids or made dull in any convenient manner, but even theupper side may be made dull. By treating the lower surface or, ifwanted, also the upper surface of the supports so as to get a highconstant of radiation, any suitable proportion between the heat suppliedfrom above and that supplied from below may be obtained.

For effecting the heat insulation of the supports from the wall andother heat conducting parts of the apparatus any convenient insulatingmaterial may be used, such as wood, board, wood fibres, asbestos,Bakelite or the like.

In the apparatus shown it is assumed that the heating is effected byelectric current. 'I'he supports 5 and heating elements 4 are carried bya frame 30 (Fig. 2) of insulating material provided with insulatedprojections 9 on which said l supports and elements rest. n the outersides of said frame there are placed electric conductors 32 with whichthe heating elements are connected. The heating elements may be of anyconvenient construction, such as plates, bands, wires. The main linesand the terminals of the elements are electrically insulated from themetal parts ofthe apparatus.

The heating elements 4 are placed at a suitable distance fromthesupports for the materials to be dried so that the heat transmittedto the latter is only radiant heat.

As shown in Fig. 1, -a cooling battery I0 is placed near the one end ofthe evacuated chamber. This battery consists of a number of parvalleltubes (see Fig. 2) placed close alongside each other, through which coldwater ilows, the temperature of the water being such that ef- .fectivecooling and consequently condensation of the water vapour is caused sothat the latter is quickly removed from the evaporation zone.

I2 is a. suction pipe leading to the vacuumpump I3. As this pipe leavesthe apparatus on the side of the cooling batteries I0 in the evaporatingchamber opposite to that on which the supports and heating elements areplaced, no water-vapour escapes from said chamber into the suction pipeof the vacuum pump as it is condensed when it comes into contact withthe cooling batteries. Consequently, it is possible to maintain a veryhigh vacuum which is indicated by the gauge I4. Thus, the vacuum pump isnot used for removing the water-vapour but only for maintaining therequisite vacuum.

The condensed water formed when the watervapour comes into contact withthe cooling batteries is collected in a recess I5 in the bottom of theoolingzone and is thence removed through the pipe I6 to the containerI1, which may suitably be graduated so that the quantity of condensedwater may be measured, thus giving a. check on the progress of thedrying process. On the pipe I6 there is a valve I8 and on the outletpipe I9 a valve 26. When water is to be removed from the vessel I1during the process, the valve I8 is closed and the valve 20 is opened.

I claim:

Apparatus for drying substances in vacuo which are liable to changeunder the action of heat, comprising a casing enclosing an evaporatingchamber, means for creating a vacuum in said chamber, metal supports forthe products to be dried in said chamber, said supports being made blackor dull on at least the lower side so as to get a. high intensity ofradiation, radiant heating elements in said chamber placed o n bothsides of each support and at a distance from the same, means forinsulating the said supports from heat conducting parts of theapparatus, and means for cooling the vapour generated from saidproducts.

